He was kind of just a victim of bad circumstances. He was love deprived and didnt have a support system. In that situation, his inferioroty complex and envy towards his richer, smarter and more loved cousin kinda got out of hand. Keep in mind he was like. 16? 17? Back then.
After that, i guess it was traumatized dudes being traumatized dudes, he just shut off the part of his brain that said it was wrong till siwon knocked some sense into him lol
He was so badly abused both emotionally and physically he couldn’t love himself, so he took out his self hatred on someone else, namely Daun, who he envies. In his redemption arc you can tell that he actually wants a relationship with Daun, not the fake nice guy he was before, but of a true cousin, teasing and poking at each other, but with no malice or ill intentions bc he’s no longer tied down by his family. Honestly what he did to Daun wasn’t terrible or unforgivable, if anything the Jiyun girl (who bullied Siwon) was so much worse, which is why I’m glad she was never forgiven, because she always had a terrible personality.

I don't feel bad for Kyungjin I was actually kinda hoping he succeeded in his self assassination. I don't how get his whole "redemption" arc was supposed to make me like him. If someone could please explain to me why I'm supposed to feel bad for him it would be gladly appreciated